Optical polarizing films have found wide use in a variety of industries. Many polarizers have been investigated and there are polarizers based on polymers, for example polyvinyl alcohol, which are made optically anisotropic by uniaxial stretching of a thin film of such polymer. The onset of optical anisotropy is due to stretching of the polymer molecules and their subsequent orientation along the direction of stretching as described in U.S. Pat. No. 5,007,942.
Upon exposure to iodine vapor, iodine-containing solution or an organic dyestuff, the film is colored. The intensity of the color depends on the direction of the electric field vector of the electromagnetic wave E relative to the axis of stretching.
The polarizing efficiency of such films is determined by the concentration of iodine or other dyestuff in the polymer film and the degree of orientation of the polymer molecules.
Despite the high polarizing efficiency of polarizers obtained from currently commonly used materials, they have a substantial disadvantage in that two cross-oriented polarizers have a high transmittance of light incident at an azimuth angle, especially at azimuth of ±45° relative to the axis of polarization.
Another prior art polarizer is based on sulfo-acids of aso- and polycyclic compounds or their mixtures as well as their salts as described in WO 94/28073. Solution of the known material can form a stable lyotropic liquid-crystal phase, which allows manufacturing of optically anisotropic films. In order to obtain a film based on this known material, the lyotropic liquid-crystal dyestuff is applied and oriented on the substrate. The solvent is later vaporized and a thin film of ordered molecules remains on the substrate. The structure of the film is comprised of ordered complexes of flat molecules of the known material. The planes of molecules and their dipole moments of optical transition are oriented perpendicular to the axis of macroscopic orientation in the film. Creation of such structure makes use of the liquid-crystal condition of the dyestuff where the molecules are already locally oriented, while existing in one- or two-dimensional quasi-crystal groups, oriented relative to each other. Upon application of such structure along with the simultaneous external alignment force, it assumes macroscopic orientation, which will not only remain, but could also improve due to the effect of crystallization during drying process. The resultant axis of polarization is along the direction of the application.
Polarizers based on this material have a number of disadvantages, which limit its applicability. In particular, it has insufficient polarizing efficiency and low angular characteristics. This results in the fact that when used in various devices, which use two parallel polarizers of herein described type, there is some undesirable transmittance of unpolarized light incident at an angle to their surface. This effect is especially prominent when one of the polarizers has diffuse-reflective coating, which is used in most of liquid-crystal displays. One of the reasons of the above disadvantages is the fact that there is no control over the degree of absorption along the normal to the plane of the polarizer. Therefore the ratio of the coefficients of absorption along the two axes, one of which lays in the plane of the substrate and perpendicular to the direction of the orientation and the other coincides with the normal to the substrate, is not optimized. Accordingly, it is desirable to provide improved polarizers.